Started reloading the x39 recently after about 20 years. My problem is the neck expander doesn’t expand enough to be able to start a .310 bullet much less a .308 bullet. I never experienced this in the past. Would appreciate any advice.

A Lee neck expander die should solve the problem. It comes with two mandrels to use from 22 cal up thru 45 think. Basically will let you bell the mouth of the case like you do on straight cases.
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Sounds like there may be more to your story and equipment, especially if you're using jacketed bullets. For cast bullets (and it would work for jacketed as well), I use a .375 expander and very slightly bell the case mouth (just enough to start the bullet) for all .30 calibers. Other diameter expanders, from .32" to .357" should also work fine.

However, if your dies worked okay twenty years ago, seems like they would work now. If you can get a bullet started and seated, check your neck diameter with a good micrometer. A caliper won't work as well if you want an accurate reading. Good luck-

If you have calipers or a mic, measure the expander you currently have.
RCBS & LEE both make die sets that come with a .306”/.307” (for use with a .308” bullet) & .309/.310” (for use with a .310/.311” bullet) neck expander for use with the resizing die. You can get a set of the expander stems from Midwayusa or even Amazon. Not expensive, less than $5.00. My problem is keeping the brass cases id’ed and separated.

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I use RCBS dies for my CZ457 which calls for the .310. Right offhand I cannot recall if more than one expander in in the box but I have never encountered any difficulty seating .310 bullets.
Is there any chance in the 20 year hiatus you may have swapped out the expander ?

My Lee die set only came with one expander, but it seems to work for .308 as well as .310 bullets. My only thought is have you chamfered your case mouths? I can't imagine why a die set would quit working after 20 years of disuse unless, as twodog max suggested, the expander has wandered off.

This is exactly what I thought. I have a Lee set and used the .311 bullets with no problems.

Maybe use some neck lube?

Is their anything different about the brass??? It's not steel cased is it?

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The last I knew, the RCBS 7.62x39 dies come with both expanders - one for 0.308" and one for 0.310".

But it was many years ago when I got them, so it may have changed,

Along the same line of expander problems, Does the stem actually have a expander ball on it, as opposed to just the threaded cap to hold the decapping pin? I'm assuming you have RCBS dies, you didn't say and it will make a difference.

Ivan

Both the RCBS and Lee 7.62 x 39 dies come with two case neck expanders, one for .308" and .310" dia. bullets. The .310 expander ball or rod depending on the die set you have will work just fine for .311" dia. jacketed bullets. On the occasions that I or my brother do load a jacketed bullet larger than .308" the provided .310" expander has worked just fine.

There are some options if the standard expanders that come with several die sets, just don't cut it for you. One is using two-step case mouth expanders made by Lyman or NOE, those are two that I know make a two-step expander for 7.62 x 39. The size you choose depends on the dia. jacketed or cast bullet you are going to use. The Lyman version is called the M die, using either the M1 or M2 die body and the appropriate two-step expander plug which best matches your bullet's dia. on the list. Lyman pistol dies already come with a two-step expander, while you will need to purchase the die body and expander or expanders for rifle calibers.

NOE also makes both a solid and a powder through version of the two-step expander that is a drop in fit using the Lee Universal Flaring die, you just use the NOE expanders in place of the ones provided by Lee, to get the same effect as using the Lyman M die. I use both the solid and powder through versions for all my cast bullet rifle loads. There are two numbers associated with the expanders, the first number which is the largest expands the case mouth, the second number expand the case neck. In general, depending on the bullet's dia. say I'm loading a .314" cast bullet in 7.62 x 39 I want my expander to be .002" larger on the first step and .002" smaller for the case neck dia. so the expander I would choose would be .316" x .312" NOE has lots of options on these expanders so it's your choice as to what dia. you choose.